Ciliophora

Abstract Ciliophora is the name for a phylum of protists commonly called the ciliates. Ciliates are the most complex of cells, having an elaborate cytoskeleton, cilia and two different kinds of nuclei. Free‐living ciliates can be found in almost any habitat that has water – in soils, hot springs and Antarctic sea ice. Symbiotic species live as commensals in sea urchins or as parasites of lobsters and fish. Ciliate life histories can have specialised forms for dispersal and for resisting desiccation. Their cell cortex is supported by a complex framework of basal bodies or kinetosomes, microtubules and microfilaments. The kinetosomes form the central unit in an organellar structure called the kinetid, which is important to understanding phylogenetic relationships among ciliates. The pattern of fibres and microtubules in the kinetid identifies a ciliate to a major clade, along with the sequences of genes. These two features together identify 12 major clades or classes of ciliates. Key Concepts: Ciliates are characterised by three main features: they exhibit nuclear dimorphism; undergo conjugation as a sexual process; and typically have cilia at some stage in their life cycle. Ciliates are the ‘top’ predators in microbial food webs, and were likely the major predatory group before the evolution of animals. Parasitic ciliates can cause morbidity and death of animals, and are becoming particularly important in aquaculture operations. Ciliates can have complex life cycles, including macrostome or cannibalistic stages, swarmers or dispersal stages and cyst or desiccation‐resistant stages. The kinetid, an organellar complex in the cell cortex, is composed of at least one kinetosome and its cilium associated with two microtubular ribbons and a striated kinetodesmal fibril, whose patterned arrangement identifies a ciliate to a particular major clade or class. Ciliate macronuclei divide in two ways, which suggest that macronuclear division evolved independently twice in the phylum: heterotrich ciliates divide their macronucleus principally using extramacronuclear microtubules while intramacronucleate ciliates divide their macronucleus with intramacronuclear microtubules. Ciliates are divided into two major clades or subphyla and 12 classes based on features of the kinetid and sequences of genes.